Unit #1



Activity 16
We Have to Stick Together ... We’re Molecules



Activity Overview

There is an attractive force between all atoms, causing or preventing phase changes and resulting in specific melting and boiling points.

Class begins with a discussion about the necessity of changing the model to accommodate attractive forces between atoms and why this is needed to model phase changes. Students then use a computer model to visualize phase changes and understand boiling points (due to van der Waals attractions).

Learning Objectives

Students will:

Conceptual Prologue

Macro-Micro Connection

Even on the nicest day a hot air balloonist often encounters clouds. Clouds consists of many tiny liquid drops. This activity will help explain how and why liquids form.

Other macro connections:

Science Concepts

Before this activity, gasses, almost exclusively, have been modeled. The model has assumed there is only an interaction between atoms when they collide. If this were true, then there would be no way for anything to exist in a liquid or solid state. Everything would be a gas! To model liquids and solids another set of rules needs be added to make the model more accurate:

When atoms are moving around fast enough (meaning they are hot enough), they then bounce off of each other with enough force to overcome the attraction felt between them. If you cool them down enough, they slow down, having less kinetic energy [energy of motion], and start sticking together. They are unable to bounce off of each other because the attractive force holds them close. A substance in this state is called a liquid. The change from gas to liquid is called condensation, the opposite change, boiling.

In the liquid state, atoms are not bouncing around freely, but they don’t form rigid structures either. The atoms have enough energy to flow by each other, continuously changing which atoms are next to each other. If the substance is cooled even more, the atoms will form a more ordered structure with little more than vibrational movement. Substances that form a more rigid, ordered structure are considered to be solids. The change from liquid to solid is called freezing, the opposite change, melting. The process of changing directly from solid to gas is called sublimation.

Depending on the strength of the attractive force between atoms of a substance the temperature at which liquids and solids form will vary. For example, nitrogen molecules (made of two atoms) have a very weak attractive force, so nitrogen boils at -196°C. Water molecules (made of three atoms) have a much stronger attractive force between them so water boils at 100°C.

Consider the process by which a substance melts and then boils. As a solid the atoms will be well ordered, vibrating around relatively fixed positions. When they are heated, they begin to vibrate even more, until, when the temperature gets high enough, the atoms start to slide by each other. At this temperature they still don’t have enough energy to completely break free of their attractions so the substance melts, becoming fluid, easily changing shape, with atoms still being held close together. If more heat is added, the atoms start to move even faster, eventually bouncing off each other with enough force to break free of the attraction, boiling, and becoming a gas.

Naive Conceptions

• The attraction between atoms gets weaker when a substance melts or boils.
The attraction is determined only by the distance between and type of atoms. Often students think that as you heat a liquid, the attractions between the atoms get weaker, so they separate from each other and form a gas. Instead, the attraction remains constant. As the motion of the atoms increases with increasing temperature, they eventually bounce off of each other with such force that they break free of the attractions. Every time two gaseous atoms come close together they feel this attraction just as strongly as if they were in the liquid or solid state. However, if they collide with enough force they just break free of this attraction and remain gaseous.

Activity Design and Execution

Major Science Concepts: • Phase change
• Weak interatomic forces (van der Waals forces or London Dispersion forces)
Assumed Previous Knowledge: • That molecules are in continual motion.
• That the kinetic energy [energy of motion] of molecules is related to their temperature.
• That higher temperature results in higher kinetic energy so molecules move faster.
• How to recognize when a bunch of molecules represents a solid, liquid, or gas.
Time: • Part A: approximately 25 minutes
• Part B: approximately 20 minutes
• Part C: approximately 25 minutes (without the liquid nitrogen demo)
Materials: • Computers with Workbench software.

For the demonstrations:
• Liquid nitrogen.*
• Rubber racquet balls.*
• Other soft things to put into liquid nitrogen.*

* This can be done as a fun extension if you can get the materials.
Advanced Preparation: (if any) • None

Investigative Question: Why do solids and liquids form and why at particular temperatures?

 

  1. Ask them if everything boils at the same temperature? [They should say, "No".] Tell them that the next simulation will help them explore their predictions about why substances have different boiling and freezing points.
  2. Using Workbench software have them bring up the "Changing Phase" activity, complete this, and then bring up the "van der Waals" activity.
  3. It will then ask them to turn the gas into a liquid and indicate when they think they have formed a liquid. Depending on the state of the model, the computer will verify their settings and ask them to try again or allow them to proceed.
  4. The software will then ask them to create a solid and indicate when they think they have formed this state.
  5. The software will then ask them to explain why the gas forms into a liquid and solid.
  6. The software will prompt the students to try atoms of differing attractive force
  7. The software will then present the student with several atoms of known attractive force and ask the students to predict which will have the highest boiling point. They will then verify their prediction.
  8. Have a class discussion on what students learned while using the software. Ask them if the computer model behaved as they expected it to. If not, in what ways did it behave differently.
  9. If you have the supplies, do some fun demonstrations of freezing things solid with liquid nitrogen. After dipping a racquetball in liquid nitrogen for a couple of minutes you can smash it on the floor. Note: Be sure to wear safety glasses and don’t touch the ball directly due to its extremely cold temperature.

Assessment

Have students write several things in their notebooks:

    1. Clouds which you are likely to see while hot air ballooning are formed because gaseous water in the air cools enough to form tiny liquid droplets. Imagine that you are sitting on a water molecule which behaves like a single atom and you are floating around in the air. Describe what you would see and feel as you floated high into the atmosphere where the temperature is lower and you begin to encounter other water molecules in the process of forming a cloud.
    2. When it snows, the liquid drops in clouds freeze solid. Describe on atomic level the difference between a liquid and a solid.
    3. Liquid nitrogen boils at a frigid -80°C while water boils at 100°C. Which substance has stronger attractions between its atoms? Why?
    4. Some cooking pans are advertised as "nonstick". Think about what you learned regarding differences in atomic attractions. What must be true about the atoms lining nonstick pans to give them the property of "non-stickiness"?
Extensions
• None
Additional Resources
• None


Internal Notes:
• See computer labs K, L, and M for mock ups of these activities.